1. Field of the Invention
This invention relates to mutant proteolytic enzymes having improved properties relative to the wild-type enzyme, to genetic constructs which code for the mutant proteolytic enzymes, to methods of predicting mutations which enhance the stability of the enzyme, and to methods of producing the mutant proteolytic enzymes.
2. Description of the Related Art
Subtilisins are a family of extracellular proteins having molecular weights in the range of 25,000-35,000 daltons and are produced by various Bacillus species. These proteins function as peptide hydrolases in that they catalyze the hydrolysis of peptide linkages in protein substrates at neutral and alkaline pH values. Subtilisins are termed serine proteases because they contain a specific serine residue which participates in the catalytic hydrolysis of peptide substrates. A subtilisin enzyme isolated from soil samples and produced by Bacillus lentus for use in detergent formulations having increased protease and oxidative stability over commercially available enzymes under conditions of pH 7 to 10 and at temperature of 10 to 60.degree. C. in aqueous solutions has been disclosed in copending patent application Ser. No. 07/398,854, filed on Aug. 25, 1989. This B. lentus alkaline protease enzyme (BLAP, vide infra) is obtained in commercial quantities by cultivating a Bacillus licheniformis ATCC 53926 strain which had been transformed by an expression plasmid which contained the wild type BLAP gene and the B. licheniformis ATCC 53926 alkaline protease gene promoter.
Industrial processes generally are performed under physical conditions which require highly stable enzymes. Enzymes may be inactivated by high temperatures, pH extremes, oxidation, and surfactants. Even though Bacillus subtilisin proteases are currently used in many industrial applications, including detergent formulations, stability improvements are still needed. Market trends are toward more concentrated detergent powders, and an increase in liquid formulations. Increased shelf stability and oxidative stability, with retention of catalytic efficiency are needed. It is therefore desirable to isolate novel enzymes with increased stability, or to improve the stability of existing enzymes, including subtilisin proteases such as BLAP.
The stability of a protein is a function of its three dimensional structure. A protein folds into a three dimensional conformation based upon the primary amino acid sequence, and upon its surrounding environment. The function and stability of a protein are a direct result of its three dimensional structure.
A large body of information has been published which describes changes in enzyme properties as a result of alterations in the primary amino acid sequence of the enzyme. These alterations can result from random or site specific alterations of the gene which expresses the enzyme using genetic engineering techniques. Random approaches mutagenize total cellular DNA, followed by selection for the synthesis of an enzyme with improved properties. This approach requires neither knowledge of the three dimensional structure of the enzyme, nor any predictive capability on the part of the researcher. Site directed mutagenesis, on the other hand, requires a rational approach for the introduction of amino acid changes. In this approach one or more amino acids may be replaced by other residues by altering the DNA sequence which encodes the protein. This can be accomplished using oligonucleotide directed in vitro mutagenesis. The following references teach site-directed mutagenesis procedures used to generate specific amino acid substitution(s): Hines, J. C., and Ray, D. S. (1980) Gene 11:207-218; Zoller, M. J., and Smith, M. (1982) Nucleic Acids Res. 10:6487-6500; Norrander, J., et al. (1983) Gene 26:101-106; Morinaga, Y., et al. (1984) Bio/Technology 2:636-639; Kramer, W., et al. (1984) Nucleic Acids Res. 12:9441-9456; Carter, P., et al. (1985) Nucleic Acids Res. 13:4431-4443; Kunkel, T. A. (1985) Proc. Natl. Acad. Sci. USA 82:488-492; Bryan, P., et al. (1986) Proc. Natl. Acad. Sci. USA 83:3743-3745.
A rational approach may or may not require knowledge of a protein's structure. For example, patent application WO 89/06279 describes the comparison of the primary amino acid sequence of different subtilisins while contrasting differences in physical and chemical properties. The primary amino acid sequences of the different subtilisins are aligned for the greatest homology, while taking into account amino acid insertions, deletions, and total number of amino acids.
Currently, the amino acid sequences of at least 10 subtilisin proteases have been published. Eight of these subtilisins were isolated from species of Bacilli, and include subtilisin 168 (Stahl, M. L., and Ferrari, E. (1984) J. Bacteriol. 158:411-418), subtilisin BPN' (Vasantha, N., et al., (1984) J. Bacteriol. 159:811-819), subtilisin Carlsberg (Jacobs, M., et al. (1985) Nucleic Acids Res. 13:8913-8926), subtilisin DY (Nedkov, P., et al. (1985) Biol. Chem. Hoppe-Seyler 366:421-430), subtilisin amylosacchariticus (Kurihara, M., et al. (1972) J.Biol. Chem. 247:5619-5631), subtilisin mesenticopeptidase (Svendsen, I., et al. (1986) FEBS Lett. 196:228-232), subtilisin 147 and subtilisin 309 (Hastrup et al. (1989) WO 89/06279), subtilisin PB92 (Van Eekelen et al. (1989) EP 0328229), and subtilisin BLAP (Ladin, B., et al. (1990) Society for Industrial Microbiology Annual Meeting, Abstract P60). The remaining two subtilisin sequences are thermitase from the fungus Thermoactinomyces vulgaris (Meloun, B., et al. (1985) FEBS Lett. 183:195-200), and proteinase K from the fungus Tritirachium album limber (Jany, K.-D., and Mayer, B. (1985) Biol. Chem. Hoppe-Seyler 366:485-492).
Methods for obtaining optimum alignment of homologous proteins are described in Atlas of Protein Sequence and Structure, Vol. 5, Supplement 2 (1976) (Dayhoff, M. O., ed., Natl. Biomed. Res. Found., Silver Springs, Md.). This comparison is then used to identify specific amino acid alterations which might produce desirable improvements in the target enzyme. Wells, J. A., et al. (1987) Proc. Natl. Acad. Sci. USA 84:1219-1223, used primary sequence alignment to predict site directed mutations which affect the substrate specificity of a subtilisin. Using the alignment approach WO 89/06279 teaches the construction of mutant subtilisins having improved properties including an increased resistance to oxidation, increased proteolytic activity, and improved washing performance for laundry detergent applications. Patent applications WO 89/09819, and WO 89/09830 teach improvement in the thermal stability of subtilisin BPN' by the introduction of one or more amino acid changes based on the alignment of the primary amino acid sequences of subtilisin BPN' with the more thermal stable subtilisin Carlsberg. From hereon, amino acids will be referred to by the one or three letter code as defined in Table 1.
TABLE 1 ______________________________________ One and Three Letter Code for Amino Acids ______________________________________ A = Ala = Alanine C = Cys = Cysteine D = Asp = Aspartic acid or aspartate E = Glu = Glutamic acid or glutamate F = Phe = Phenylalanine G = Gly = Glycine H = His = Histidine I = Ile = Isoleucine K = Lys = Lysine L = Leu = Leucine M = Met = Methionine N = Asn = Asparagine P = Pro = Proline Q = Gln = Glutamine R = Arg = Arginine S = Ser = Serine T = Thr = Threonine V = Val = Valine W = Trp = Tryptophan Y = Tyr = Tyrosine ______________________________________
Rational mutational approaches may also predict mutations which improve an enzyme property based upon the three dimensional structure of an enzyme, in addition to the alignment of primary amino acid sequences described above. One method for determining the three dimensional structure of a protein involves the growing of crystals of the protein, followed by X-ray crystallographic analysis. This technique has been successfully used to determine several high resolution subtilisin structures such as thermitase (Teplyakov, A. V., et al. (1990) 214:261-279), subtilisin BPN' (Bott, R., et al. (1988) J. Biol. Chem. 263:7895-7906) and subtilisin Carlsberg (Bode, W., et al. (1986) EMBO J. 5:813-818), for example.
EP 0251446 teaches the construction of mutant carbonyl hydrolases (proteases) which have at least one property different from the parental carbonyl hydrolase. It describes mutations which effect (either improve or decrease) oxidative stability, substrate specificity, catalytic activity, thermal stability, alkaline stability, pH activity profile, and resistance to autoproteolysis. These mutations were selected for introduction into Bacillus amyloliquefaciens subtilisin BPN' after alignment of the primary sequences of BPN' and proteases from B. subtilis, B. licheniformis, and thermitase. Such alignment can then be used to select amino acids in these other proteases which differ, as substitutes for the equivalent amino acid in the B. amyloliquefaciens carbonyl hydrolase. This application also describes alignment on the basis of a 1.8 .ANG. X-ray crystal structure of the B. amyloliquefaciens protease. Amino acids in the carbonyl hydrolase of B. amyloliquefaciens which when altered can affect stability, substrate specificity, or catalytic efficiency include: Met50, Met124, and Met222 for oxidative stability; Tyr104, Ala152, Glu156, Gly166, Gly169, Phe189, and Tyr217 for substrate specificity; N155 alterations were found to decrease turnover, and lower Km; Asp36, Ile107, Lys170, Asp197, Ser204, Lys213, and Met222 for alkaline stability; and Met199, and Tyr21 for thermal stability. Alteration of other amino acids was found to affect multiple properties of the protease. Included in this category are Ser24, Met50, Asp156, Gly166, Gly169, and Tyr217. Substitution at residues Ser24, Met50, Ile107, Glu156, Gly166, Gly169, Ser204, Lys213, Gly215, and Tyr217 was predicted to increase thermal and alkaline stability. An important point about this patent application is that with the exception of those mutations effecting substrate specificity, no rational mutational approach for improving the alkaline or temperature stability of a protease based upon computer simulations of an X-ray crystal structure is described.
WO 88/08028 teaches a method for redesigning proteins to increase stability by altering amino acid residues that are in close proximity to the protein's metal ion binding site. This application describes the alteration of a calcium ion binding site present within subtilisin BPN' through the substitution, insertion, or deletion of amino acid residue(s) in close proximity to that site so that the electrostatic attraction between the amino acids and the calcium ion is increased. The characterization of the calcium ion binding site is accomplished through the analysis of a 1.3 .ANG. three dimensional structure of subtilisin BPN' using a high resolution computer graphics system. This approach allows the selection of amino acids acceptable for replacing the native amino acids in the protease by first simulating the change using the computer model. This allows for the identification of any problems including steric hindrance prior to the actual construction and testing of the mutant proteases.
U.S. Pat. Nos. 4,908,773 and 4,853,871 teach a computer based method for evaluating the three dimensional structure of a protein to select amino acid residues where the introduction of a novel disulfide bond will potentially stabilize the protein. Potentially acceptable amino acid residues can then be ranked, and replaced using computer simulation, prior to the actual construction of the mutant protein using site directed mutagenesis protocols.
Several patent applications combine published data on biochemical stability with computer analysis of three dimensional protease structures in order to predict mutations which stabilize the enzyme. U.S. Pat. No. 4,914,031 and WO 88/08033 and WO 87/04461 teach a method for improving the pH and thermal stability of subtilisin aprA by replacing asparagine residues present in asparagine/glycine pairs. Asparagine/glycine pairs in proteins have been shown to undergo cyclization to form cyclic imide anhydroaspartylglycine (Bornstein, P., and Balian, G. (1977) Methods Enzymol. 47:132-145). This cyclic imide is susceptible to base hydrolyzed cleavage leading to inactivation of the enzyme. Computer analysis of the three dimensional structure of the aprA protease also predicted that formation of the cyclic imide could lead to protease inactivation resulting from a shift of the side chain of the active site serine. The decision to replace the asparagine residue and not the glycine residue was based upon alignment of the aprA sequence with other subtilisin-like enzymes, cucumisin and proteinase K.
Sensitivity to oxidation is an important deficiency of serine proteases used in detergent applications (Stauffer, C. E., and Etson, D. (1969) J. Biol. Chem. 244:5333-5338). EP 0130756, EP 0247647, and U.S. Pat. No. 4,760,025 teach a saturation mutation method where one or multiple mutations are introduced into the subtilisin BPN' at amino acid residues Asp32, Asn155, Tyr104, Met222, Gly166, His64, Ser221, Gly169, Glu156, Ser33, Phe189, Tyr217, and/or Ala152. Using this approach mutant proteases exhibiting improved oxidative stability, altered substrate specificity, and/or altered pH activity profiles are obtained. A method is taught in which improved oxidative stability is achieved by substitution of methionine, cysteine, tryptophan, and lysine residues. These publications also teach that mutations within the active site region of the protease are also most likely to influence activity. Random or selected mutations can be introduced into a target gene using the experimental approach but neither EP 0130756, EP 0247647, nor U.S. Pat. No. 4,760,025 teach a method for predicting amino acid alterations which will improve the thermal or surfactant stability of the protease.
WO 8705050 teaches a random mutagenesis approach for construction of subtilisin mutants exhibiting enhanced thermal stability. One or more random mutations are introduced into single stranded target DNA using the chemical mutagens sodium bisulfite, nitrous acid, and formic acid. Subsequently, the mutated DNA is transformed into a Bacillus host and at least 50,000 colonies are screened by a filter assay to identify proteases with improved properties. Site directed mutagenesis can then be used to introduce all possible mutations into a site identified through the random mutagenesis screen. No method for pre selection of amino acids to be altered is taught.
EP 0328229 teaches the isolation and characterization of PB92 subtilisin mutants with improved properties for laundry detergent applications based upon wash test results. It teaches that biochemical properties are not reliable parameters for predicting enzyme performance in the wash. Methods for selection of mutations involve the substitution of amino acids by other amino acids in the same category (polar, nonpolar, aromatic, charged, aliphatic, and neutral), the substitution of polar amino acids asparagine and glutamine by charged amino acids, and increasing the anionic character of the protease at sites not involved with the active site. No method for identifying which specific amino acids should be altered is taught, and no rational mutational approach is taught which is based on alignment of X-ray structures of homologous proteases with different properties.
EP 0260105 teaches the construction of subtilisin BPN' mutants with altered transesterification rate/hydrolysis rate ratios and nucleophile specificities by changing specific amino acid residues within 15 .ANG. of the catalytic triad. Russell, A. J., and Fersht, A. R. (1987) Nature 328:496-500, and Russell, A. J., et al. (1987) J. Mol. Biol. 193:803-813, teach the isolation of a subtilisin BPN' mutant (D099S) that had a change in the surface charge 14-15 .ANG. from the active site. This substitution causes an effect on the pH dependence of the subtilisin's catalytic reaction.
There are a number of different strategies for increasing protein stability. Many of these methods suggest types of substitutions to improve the stability of a protein but do not teach a method for identifying amino acid residues within a protein which should be substituted. From entropic arguments, many types of substitutions have been suggested such as Gly to Ala and any amino acid to Pro (Matthews, B. W., et al. (1987) Proc. Natl. Acad. Sci. 84:6663-6667). Likewise, while it is clear that increasing the apolar size of an amino acid in the core will add to stability, adverse packing effects may more than compensate for the hydrophobic effect, resulting in a decrease in protein stability (Sandberg, W. S., and Terwilliger, T. C. (1989) Science 245:54-57). Menendez-Arias, L., and Argos, P. (1990) J. Mol. Biol. 206:397-406, performed a statistical evaluation of amino acid substitutions of thermophilic and mesophilic molecules and proposed that decreased flexibility and increased hydrophobicity in the .alpha.-helical regions contributes most towards increasing protein stability. From their data, they formulated a set of empirical rules to improve stability.
Increasing the hydrophobicity of certain side chains has long been suggested as a means to improve protein stability. The hydrophobic exclusion of nonpolar amino acids is the largest force driving protein folding. This has been studied by examining the partitioning of amino acids or amino acid analogs from water to a hydrophobic medium. While the numbers vary depending on the work, these studies generally agree that burying a hydrophobic side chain increases protein stability. For example, Kellis, J. T., Jr., et al. (1988) Nature 333:784-786, estimated that the removal of a methyl group destabilizes the enzyme by 1.1 kcal/mole assuming no other structural perturbations occur. Conversely, this predicts that the addition of a methylene group should add 1.1 kcal/mol if no unfavorable contacts occur. Similarly, Sandberg, W. S., and Terwilliger, T. C. (1989) Science 245:54-57, showed that the effect of removing or adding methylene groups is the sum of the hydrophobic effect and structural distortions. Simply adding buried hydrophobic groups may not increase protein stability because the total effect of adding or deleting a methyl group on the local packing structure must be considered. As the protein interior has a para-crystalline structure (Chothia, C. (1975) Nature 254:304-308), small distortions in the remainder of the structure resulting from the addition methyl group may exact a high cost and reduce rather than increase stability.
Along the same lines, the core of .lambda. repressor has been shown to be amazingly tolerant to apolar amino acid substitutions in a functional assay (Bowie, J. U., et al. (1990) Science 247:1306-1310). It is not clear that this is true for larger proteins. The constraints on the hydrophobic core of a small protein may be less stringent than a larger protein simply due to the volume of the core relative to the number of amino acids which need to pack into the region. As the volume of the hydrophobic core increases, the number of amino acids which must pack together correctly increases, requiring more specific nonlocal interactions.
It has been recognized that increasing the interior hydrophobicity of a protein as a means of increasing the stability is hampered by the difficulty of determining which positions in the protein will lead to stabilization when substituted (Sandberg, W. S., and Terwilliger, T. C. (1991) Trends Biotechnol. 9:59-63). The methods discussed above provide a means of determining what substitutions to make to improve stability but do not identify which sites in the protein are most important. The present invention provides a method of determining which positions in the protein will lead to stabilization when substituted.